The focus distance scales on lenses used with underwater cameras need to be re-calibrated if sharp underwater images are to be obtained. This is normally best done in a swimming pool where a diver can have more control, and external factors are less likely to intrude. The first step is to set up a focus chart on a tripod at the bottom of a pool, and then place the camera, also on a tripod, just a few feet in front of the chart. The chart should be well lit, ideally with two sources placed at a 45 degree angle either side of the camera-to-chart line. Measure the distance between the focus chart and the camera's image plane, focus the lens by eye, and then mark the relevant distance setting on the focus scale with a pencil. This process should be repeated at numerous distances up to about 30 feet or more. It should also be carried out for each lens intended for underwater use. Once the underwater calibrations have been completed, permanent marks must be established on all the lenses so the appropriate settings can be found during a dive. There is no doubt that a calibration process such as this is time consuming and requires a good deal of preparation, but the rewards in terms of properly focused images make the effort worthwhile.
Two types of underwater port are commonly used. These are the flat and domed ports. Hemispherical domed ports are normally the best choice. However they cannot be used above water and may not be appropriate for extreme close-up shots. Flat ports can be used above water but produce various types of aberration when used underwater. These include refraction, chromatic aberration and radial distortion. Light rays are not distorted uniformly by flat ports, so a progressive radial distortion is produced in images and becomes more apparent at wider angles of view. Radial distortion appears as a progressive blur which increases with large apertures because light rays passing through the centre of the port are perpendicular to its surface and consequently not affected. Rays passing through the port further from its optical centre pass through the various air, glass and water interfaces at increasingly angles and are consequently more deflected. Chromatic aberration occurs in white light because, when refracted, it is separated into the colours of the spectrum. The component colours of white light are not propagated at identical speeds, and are deflected by different amounts as they pass through the various air, glass and water interfaces. Interference between the separated colors then causes a loss of sharpness. The effect is once again more noticeable with wide-angle lenses.
With a flat port, refraction produces an apparent reduction of the subject-to-camera distance to three-quarters of the true physical distance. It is therefore often sufficient, particularly in good light when using smaller apertures, to set the lens to three-quarters of the true distance (should it be known). For example, the lens would be focused at 9 feet when photographing a subject at a distance of 12 feet. In practice, however, the actual camera-to-subject distance is unlikely to be known with any degree of accuracy. Also, when using lenses of a longer focal length or when working in poor light, it is wise to focus the lens using underwater calibration marks.
A domed port is essentially an additional concentric lens element forming part of a camera's optical system. It effectively eliminates aberrations due to refraction, radial distortion and chromatic aberrations when the centre of curvature of the dome's inner surface is co-incident with the entry nodal point of the lens. Incident rays are all then normal to the port's surface and hence not refracted. Domed ports also allow lenses designed for above-water use to retain their normal angle of view. However a domed port creates a virtual image a short distance in front of the lens upon which the lens must be focused. The lens is not focused upon the subject itself. When using a domed port, a lens should be calibrated by multiplying the inside radius of the relevant domed port by four, and then setting the lens to this distance for underwater eye focus calibration. Domed ports typically have a four-inch internal radius, so a lens should be set to 16 inches to begin calibration for underwater work. Lenses featuring a minimum focus distance less than that required may be fitted with a supplementary lens (typically plus 1 to plus3 dioptres) to solve the problem. In practice, a lens needs a range of focus extending from a minimum of about nine or ten inches to achieve sharp underwater images from one or two feet up to infinity.